The field of the disclosure is related to superconducting circuits. More particularly, the disclosure is directed to systems and methods for quantum information processing and quantum computation.
In the field of quantum computation, the performance of quantum bits (“qubits”) has advanced rapidly in recent years, with preliminary multi-qubit implementations leading toward surface code architectures. In contrast to classical computational methods that rely on binary data stored in the form of definite on/off states, or bits, methods in quantum computation take advantage of the quantum mechanical nature of quantum systems. Specifically, quantum systems are described using a probabilistic approach, whereby a system includes quantized energy levels whose state may be represented using a superposition of multiple quantum states.
Among the implementations currently being pursued, superconductor-based circuits present good candidates for the construction of qubits given the low dissipation inherent to superconducting materials, which in principle can produce coherence times necessary for performing useful quantum computations. In addition, because complex superconducting circuits can be micro-fabricated using conventional integrated-circuit processing techniques, scaling to a large number of qubits is relatively straightforward. However, scaling up from a few devices to a large-scale multi-qubit circuit presents specific challenges, particularly in the context of quantum measurement, requiring additional resources, infrastructure and complexity.
Superconducting qubits have already achieved several key milestones, including single and coupled qubit state tomography, gate fidelity in excess of 99.9%, and generation of arbitrary quantum states in a superconducting resonator. In particular, circuit Quantum Electrodynamics (“cQED”) configurations provide an attractive paradigm for scaling to large numbers of qubits. Here a superconducting qubit plays the role of an artificial atom, and a thin-film coplanar waveguide or bulk cavity resonator is used to realize a bosonic mode with strong coupling to the atom. In the limit where the qubit is far detuned from the cavity resonance, the effective cavity frequency acquires a shift that depends on the qubit state. It is therefore possible to perform quantum non-demolition (“QND”) measurement of the qubit by monitoring the microwave transmission across the cavity.
In a conventional cQED measurement, the state of the qubit is encoded in the quadrature amplitudes of a weak microwave signal that is transmitted across the readout cavity. It is possible to access these amplitudes by pre-amplifying the signal using a low-noise linear amplifier followed by heterodyne detection, where the assignment of the detected signal to the qubit |0 or |1 states is performed by subsequent post-processing and thresholding. While this approach may work well for a small number of readout channels, the required superconducting amplifiers, cryogenic semiconducting post-amplifiers, and quadrature mixers entail significant experimental overhead. That is, the amplifiers often require biasing with a strong auxiliary microwave pump tone, which must be isolated from the qubit circuit with bulky cryogenic isolators. Moreover, there is no clear path to integrating the heterodyne detector at low temperature to provide for a more compact, scalable architecture.
Specifically, present systems for measurement and control of superconducting quantum circuits typically include low-temperature systems, such as dilution refrigeration units. Such systems are equipped with frequency generators and single-sideband mixing hardware that generate and transmit electromagnetic signals to multiple superconducting circuits for purposes of measurement and control of the state of each qubit. However, such systems are limited in terms of wiring availability, as well as thermal and noise coupling to room temperature electronics. Hence, in applications involving cryogenic temperatures it is highly desirable to integrate as much of the control and measurement circuitry for a multi-qubit system as possible into the low-temperature environment in order to reduce wiring heat load, latency, power consumption, and the overall system footprint.
Given the above, there is a need for systems and methods amenable to scalable quantum computation with fewer components and reduced overhead, while capable of achieving high performance levels.